Electrophoretic multi-color display device

ABSTRACT

The electrophoretic multi-color display device has a plurality of cells ( 10, 10′, 10 ″, . . . ) with an electrophoretic medium ( 14 ) and pixel electrodes ( 11, 11′, 11″ , . . . ) for selecting a subgroup of cells. A color filter array ( 13 ) is associated with the pixel electrodes. The color filter array ( 13 ) and the pixel electrodes ( 11, 11′, 11 ″, . . . ) are provided at the same side of the electrophoretic medium ( 14 ). Preferably, the cells ( 10, 10′, 10 ″, . . . ) of the color filter array ( 13 ) are arranged according to a matrix and are disposed along lines registering with the pixel electrodes ( 11, 11′, 11 ″, . . . ). Preferably, an insulating material ( 17 ) with a relatively low refractive index is provided between the pixel electrodes ( 11, 11′, 11 ″ . . . ) and the color filter array ( 13 ). Preferably, the insulating material ( 17 ) is selected from the group formed by a fluor-polymer, a low-dielectric inorganic film and a low-dielectric nano-porous film.

The invention relates to an electrophoretic multi-color display device.

Electrophoretic display devices are based on the motion of charged,usually colored particles under the influence of an electric fieldbetween two extreme states having a different transmissivity orreflectivity. With these display devices, dark (colored) characters canbe imaged on a light (colored) background, and vice versa.Electrophoretic display devices are notably used in display devicestaking over the function of paper and are often referred to as“electronic paper” or “paper white” applications (electronic newspapers,electronic diaries).

For mobile display applications, electrophoretic display devices offeran advantageous performance including relatively low power consumptiondue to long-term image stability, relatively high white statereflectivity and contrast, and “paper-like” optics enhancing readabilityand legibility. The optical performance of these reflective displaydevices makes them relatively insensitive to ambient lighting intensityand direction. Electrophoretic display devices provide a viewing anglewhich is practically as wide as that of normal paper. The performance issuch that supplemental illumination solutions such as front lights arenot required for many devices.

Optical materials based on microencapsulated electrophoretic ink havebeen successfully integrated with traditional a-Si thin-film transistors(TFTs), a-Si TFTs built on conformable steel foils or organic TFTs.Facile mechanical integration of the material to active matrices leadsto substantial simplifications in their cell assembly process comparedto that of liquid crystal display (LCD) devices. In monochromeelectrophoretic displays devices, for example, a flexible plastic frontsheet coated with indium tin oxide (ITO) and the electrophoretic mediumis laminated directly to a thin-film transistor backplane. Afterlamination, an edge seal is applied around the perimeter of the displaydevice. In principle, no polarizer films, alignment layers, rubbingprocesses, or spacers are required.

It has been shown that many of the performance advantages of anelectrophoretic display device are preserved when a color filter arrayis properly designed and integrated into the display device. Inelectrophoretic multi-color display devices, the white and dark statereflectivities, as well as the color saturations, are relativelyinsensitive to illumination and viewing conditions.

A drawback of the known electrophoretic multi-color display devices isthat the manufacturing (assembly) of the electrophoretic multi-colordisplay devices is relatively complicated.

The invention has for its object to eliminate the above disadvantagewholly or partly. According to the invention, an electrophoreticmulti-color display device of the kind mentioned in the openingparagraph for this purpose comprises:

a plurality of cells with an electrophoretic medium,

pixel electrodes for selecting a subgroup of cells,

a color filter array associated with the pixel electrodes,

the color filter array and the pixel electrodes being provided at thesame side of the electrophoretic medium.

The inventors have had the insight, that by forming the color filterarray and the pixel electrodes at the same side of the electrophoreticmedium, the alignment of the color filter array with respect to thepixel electrodes is greatly simplified. In this manner, themanufacturing (assembly) of the electrophoretic multi-color displaydevices becomes relatively simple.

The known lay-out of an electrophoretic multi-color display device istypically formed in the following manner. On top of a (plastic) sheetprovided with a conductive layer, (a layer of) an electrophoretic mediumis coated. This (plastic) sheet provided with the electrophoretic mediumis laminated on a substrate provided with the pixel electrodes. Ingeneral, these pixel electrodes form part of a so-called active matrixsubstrate. The known electrophoretic multi-color display device isviewed form the side of the (plastic) sheet because of the preferredarrangement of the color filter array with respect to theelectrophoretic medium. In the configuration of the electrophoreticmulti-color display device according to the invention the color filterarray is on the “viewer” side of the display device.

The assembly process of the known electrophoretic multi-color displaydevice, with the pixel electrodes and the color filter array on oppositesides of the electrophoretic medium, is hindered due to the relativelyopaque nature of the electrophoretic medium. The electrophoretic mediumobscures the alignment of the color filter array with respect to thepixel electrodes.

In the electrophoretic multi-color display device according to theinvention, the pixel electrodes and the color filter array are on thesame side of the electrophoretic medium. This arrangement of the pixelelectrodes and the color filter array with respect to theelectrophoretic medium implies that the electrophoretic multi-colordisplay device according to the invention is viewed from the oppositeside as in the known display device. The electrophoretic multi-colordisplay device is viewed from the “active matrix” side. Theelectrophoretic layer may be disposed on the active matrix substrate. Ina preferred embodiment of the electrophoretic multi-color displaydevice, the electrophoretic layer is formed on a separate (plastic) filmor sheet, further comprising the common electrodes.

Normally the color filter array comprises a “red”, a “green” and a“blue” cell, which together form a so-called “pixel” of theelectrophoretic multi-color display device.

To enable undisturbed viewing of the electrophoretic multi-color displaydevice, the pixel electrodes are, preferably, translucent. Preferably,the pixel electrodes are made from indium tin oxide (ITO) or any othersuitable transparent conduction material.

The cells of the color filter array may be arranged in differentconfigurations. Preferably, the cells are arranged in the form of amatrix (with rows and columns). The cells may be individual “dots” foreach color wherein each “dot” or cell corresponds with a pixelelectrode. In another embodiment, the color are arrange along linesfollowing the rows of pixel electrodes. In a favorable embodiments ofthe electrophoretic multi-color display device according to theinvention, each cell of the color filter array registers with a pixelelectrode. In an alternative preferred embodiment of the electrophoreticmulti-color display device according to the invention the cells of thecolor filter array are disposed along lines registering with the pixelelectrodes. In addition, the color cells may be configured in variousother manners, for instance in a triangular or prismatic lay-out.

Preferably, the pixel electrodes are provided in close contact with thecells with electrophoretic medium. To this end, a preferred embodimentof the electrophoretic multi-color display device according to theinvention is characterized in that the pixel electrodes are providedbetween the cells and the color filter array. If the color filter arrayis between the pixel electrodes and the cells, there is an undesiredvoltage drop over the color filter array.

To improve reflectivity of the electrophoretic multi-color displaydevice, a layer with a low refractive index can be applied between thecolor filter array and the pixel electrodes. To this end, a preferredembodiment of the electrophoretic multi-color display device accordingto the invention is characterized in that an insulating material with arelatively low refractive index is provided between the pixel electrodesand the color filter array. Providing such a layer with a low refractiveindex ensures that light scattered back from the electrophoretic mediumenters into the substrate layer at an angle which is not lower than theangle for total internal reflection.

Preferably, the insulating material is selected from the group formed bya fluor-polymer, a low-dielectric inorganic film and a low-dielectricnano-porous film. Favorable insulating materials are silicon nitride,silicon oxide, (nano foams of) polymer and aerogel. Such materials haverefractive indices in the range from 1 to 1.4. For instance, therefractive index is approximately 1.3 for fluor-polymers.

A preferred embodiment of the electrophoretic multi-color display deviceaccording to the invention is characterized in that each of the cells isassociated with a thin-film transistor. Preferably, the color filterarray and the thin-film transistors are provided on a translucent frontwindow, elements of the color filter array being arranged adjacent tothe thin-film transistors.

A preferred embodiment of the electrophoretic multi-color display deviceaccording to the invention is characterized in that the electricalcontact between the pixel electrodes and the respective thin-filmtransistor is provided via cut-out portions in the color filter array.When the pixel electrodes are formed on top of the color filter arraywith a layer of a low-refractive index material in between, the pixelelectrodes are be contacted through via-holes, opened in the colourfilter array.

A favorable embodiment of the electrophoretic multi-color display deviceaccording to the invention is characterized in that the cell of thecolor filter array comprises a relatively low-refractive index materialwith a refractive index n_(li) in the range from 1.0≦n_(li)≦1.5. Byintroducing a material with a relatively low refractive index more lightcan be coupled out of the cell, thereby increasing the brightness of theelectrophoretic multi-color display device. Employing a low-refractiveindex material is based on the recognition that light scattered at anangle higher than the limit angle for total internal reflection can notbe coupled out of the cell. By introducing a low-refractive indexmaterial the so-called limit angle is increased enabling more light tobe issued from the cell.

Light which is incident on the cell is redistributed by diffusescattering in the known electrophoretic multi-color display device cannot escape from the front window if the angle with the normal on thesurface of the front window exceeds the limit angle, θ_(la). If theincoming light flux has a light flux I_(in), and the light that isreflected has a light flux I_(out), the relation between the incidentlight and the light that is coupled out of the cell is, in first orderapproximation, given by:

$I_{out} \leq \frac{I_{in}( {1 - {\cos( {2\theta_{la}} )}} )}{2}$In the above formula, reflection in the cell is assumed to beLambertian. The ≦-sign in the equation has been introduced to indicatethat additional Fresnel losses have been neglected. In Table I thereflected flux I_(out) under normal conditions has been calculated forvarious values of the refractive index of the front window.

TABLE I Reflected flux for various values of the refractive index of thefront window. Refractive index n Limit angle Reflected flux of frontwindow θ_(la) I_(out) 1.0 90.0° 1.00 I_(in) 1.1 65.4° 0.83 I_(in) 1.256.4° 0.69 I_(in) 1.3 50.3° 0.59 I_(in) 1.4 45.6° 0.51 I_(in) 1.5 41.8°0.44 I_(in) 1.6 38.7° 0.39 I_(in)For a front window made of glass or Poly(EthyleneTerephthalate) (PET)n≈1.5 and only 0.44×I_(in) of the incident light is reflected. For afront window made of PolyCarbonate n≈1.6 only 0.39×I_(in) of theincident light is reflected.

By introducing a low-refractive index material more light can be coupledout of the cells of the color filter array. Preferably, the refractiveindex of the low-refractive index material is n_(li)≦1.4.

A preferred embodiment of the electrophoretic multi-color display deviceaccording to the invention is characterized in that the low-refractiveindex material is selected from the group formed by a fluor-polymer, alow-dielectric inorganic film and a low-dielectric nano-porous film. Forfluor-polymers the refractive index is approximately 1.3. According toTable I the reflected flux for n≈1.3 increases to 0.59×I_(in), a gain ofa factor of 0.59/0.44=1.34 with respect to the known electrophoreticmulti-color display devices. Low-dielectric films are for instance foundin fluorides such as LiF, which has n≈1.39 or MgF₂ which has n≈1.38.Also fluosilicates are known to have a low index of refraction, e.g.MgSiF₆ has n≈1.35 and K₂SiF₆ has n≈1.34. The refractive index ofnano-porous films or aerogels can be as low as 1.05-1.1. For n≈1.1 thegain in reflected flux is approximately 2.

A preferred embodiment of the electrophoretic multi-color display deviceaccording to the invention is characterized in that the electrophoreticmedium is present in a microcapsule. Preferably, the electrophoreticmulti-color display device comprises one microcapsule per cell or onemicrocapsule per sub-cell. The charged electrophoretic particles may bepresent in a fluid between substrates, but it is alternatively possiblefor the electrophoretic medium to be present in a microcapsule, oftenalso referred to as a microencapsulated electrophoretic medium.

These and other aspects of the invention are apparent from and will beelucidated with reference to the embodiments described hereinafter.

In the drawings:

FIG. 1 shows an electric equivalent of a part of an electrophoreticmulti-color display device;

FIG. 2 shows a number of cells of an electrophoretic multi-color displaydevice according to an embodiment of the invention in cross-section, and

FIGS. 3A and 3B show cells of an electrophoretic multi-color displaydevice according to further embodiments of the invention incross-section.

The Figures are purely diagrammatic and not drawn to scale. Particularlyfor clarity, some dimensions are exaggerated strongly. Similarcomponents in the Figures are denoted by the same reference numerals asmuch as possible.

FIG. 1 very schematically shows an electric equivalent of a part of anelectrophoretic multi-color display device 1 to which the invention isapplicable. It comprises a plurality of cells 10 (only one cell is shownin FIG. 1) at the area of crossings of row or selection electrodes 7 andcolumn or data electrodes 6. The selection electrodes 7 numbered from 1to m in FIG. 1 are consecutively selected by means of a row driver 4,while the data electrodes 6 numbered from 1 to n in FIG. 1 are providedwith data via a data register 5. To this end, incoming data 2 are firstprocessed, if necessary, in a processor 3. Mutual synchronizationbetween the row driver 4 and the data register 5 takes place via drivelines 8 connected to the processor 3.

Drive signals from the row driver 4 select the electrodes via thin-filmtransistors (TFTs) 9 whose gate electrodes are electrically connected tothe selection electrodes 7 and whose source electrodes are electricallyconnected to the data electrodes 6 (referred to as active drive). Thesignal at the data electrode 6 is transferred via the TFT to a pixelelectrode 11, coupled to the drain electrode, of a cell 10. The otherelectrodes of the cell 10 are connected to, for example, ground, forexample, by means of one (or more) so-called common (counter)electrode(s) 12. In the example of FIG. 1, such a TFT 9 with a pixelelectrode 11 and a common electrode 12 is shown diagrammatically for onecell 10 only.

Thin-film transistors are not essential to the electrophoreticmulti-color display device according to the invention. When transistorsare not provided in the electrophoretic multi-color display device, thepixel electrodes are the data electrodes and the common electrodes arethe selection electrodes.

FIG. 1 very schematically shows three color elements B, G, R from acolor filter array 13. In the example of FIG. 1, the color elements B,G, R are disposed in a linear lay-out. In addition, the respective colorlines in the color filter array are disposed in the same direction asthe selection electrodes 7. The color medium in the color filter array13 can be, for example, a light-transmissive colored filter element.

The cells of the color filter array may be arranged in differentconfigurations. The cells may be individual “dots” for each colorwherein each “dot” or cell corresponds with a pixel electrode. Infurther alternative embodiment, each cell of the color filter arrayregisters with a pixel electrode. In addition, the color cells may beconfigured in various other manners, for instance in a triangular orprismatic lay-out. Alternatively, it may be advantageous to dispose therespective color lines in the color filter array along lines registeringwith the pixel electrode 11.

FIG. 2 very schematically and not to scale shows a number of cells 10,10′, 10″, . . . of an electrophoretic multi-color display deviceaccording to an embodiment of the invention in cross-section. Atranslucent front window 20 is provided with a matrix of thin-filmtransistors 9, 9′, 9″, . . . . Preferably, the front window 20 is madeof glass or Poly(EthyleneTerephthalate) (PET). In between the thin-filmtransistors 9, 9′, 9″, . . . the color filter array 13 is disposedconsisting of three color elements B, G, R Electrophoretic cell 10 isassociated with the color element B, electrophoretic cell 10′ isassociated with the color element G, and electrophoretic cell 10″ isassociated with the color element R of the color filter array 13. Threelaterally adjacent cells 10, 10′, 10″, . . . create an electrophoreticreflective color pixel. In FIG. 2 there are also shown walls 24 betweeneach of the cells 10, 10′, 10″, . . . to figuratively demarcate thesides of each cell. These side cell walls 24 are shown for illustrativepurpose and do not have to be present in the actual embodiment.

In FIG. 2, pixel electrodes 11, 11′, 11″, . . . are provided above thecolor filter array 13. Preferably, the pixel electrodes 11, 11′, 11″, .. . are translucent and are, preferably, made from indium tin oxide(ITO) or any other suitable transparent conduction material. Thetranslucent front window 20 provided with the array of the thin-filmtransistors 9, 9′, 9″, . . . forms a known active matrix substrate whichis employed in the world of liquid crystal display devices.

In the example of FIG. 2 a layer of an insulating material 17 with arelatively low refractive index is provided between the color filterarray 13 and the pixel electrodes 11, 11′, 11″ . . . . The relativelylow refractive index of the insulating material 17 improves thereflectivity of the electrophoretic multi-color display device.

Counter electrodes, the so-called common electrodes 12 are provided atthe side of the cells 10, 10′, 10″, . . . with the electrophoreticmedium 14 facing away from the pixel electrodes 11, 11′, 11″, . . . .The common electrodes 12 are provided on a back sheet 25.

Holes are provided in the insulating material 17 to enable electricalcontact between the pixel electrodes 11, 11′, 11″, . . . and therespective thin-film transistors 9, 9′, 9″, . . . . In an alternativeembodiment, the electrical contact between the pixel electrodes and therespective thin-film transistor is provided via cut-out portions in thecolor filter array.

Contrary to the known display device, the electrophoretic multi-colordisplay device according to the invention is viewed from the activematrix side of the display.

FIG. 3A shows a (single) cell of an electrophoretic multi-color displaydevice according to a further embodiment of the invention incross-section. In the example of FIG. 3, a layer of a low-refractiveindex material 27 is provided between the electrophoretic medium 10 andthe pixel electrode 11 connected to the TFT 9 which is disposed adjacentone of the three color elements (“B”) associated with the translucentfront window 20. The common electrodes 12 is provided on the back sheet25.

In an alternative embodiment, the low-refractive index material isprovided between the translucent front window and the switchingelectrode associated with the translucent front window. Preferably, thethickness of the switching electrode is less than or equal to thewavelength of the light.

By adding one or more layers of a material with low refractive index,the brightness of the electrophoretic multi-color display device isimproved. The closer the refractive index of the low-index mediummatches that of air, the more brightness improvement is realized.Preferably, the refractive index of the layer of low-refractive indexmaterial 27 is n_(li)≦1.4. Materials in the desired range of refractiveindices are available. Preferably, the low-refractive index material 27is selected from the group formed by a fluor-polymer, a low-dielectricinorganic film and a low-dielectric nano-porous film. For fluor-polymersthe refractive index is approximately 1.3 whereas the refractive indexof nano-porous films or aero-gels is approximately 1.1. Low-dielectricfilms are for instance found in fluorides such as LiF, which has n≈1.39or MgF₂ which has n≈1.38. In addition, fluosilicates are known to have alow index of refraction, e.g. MgSiF₆ has n≈1.35 and K₂SiF₆ has n≈1.34.The refractive index of nano-porous films or aerogels can be as low as1.05-1.1.

FIG. 3B shows a cell of an electrophoretic multi-color display deviceaccording to a further embodiment of the invention in cross-section. Inthe example of FIG. 3B, the cell 10 comprises a low-refractive indexmaterial 27 with a low-refractive index material (indicated in FIG. 3Bby the reference numerals “10+27”).

It should be noted that the above-mentioned embodiments illustraterather than limit the invention, and that those skilled in the art willbe able to design many alternative embodiments without departing fromthe scope of the appended claims. In the claims, any reference signsplaced between parentheses shall not be construed as limiting the claim.Use of the verb “comprise” and its conjugations does not exclude thepresence of elements or steps other than those stated in a claim. Thearticle “a” or “an” preceding an element does not exclude the presenceof a plurality of such elements. The invention may be implemented bymeans of hardware comprising several distinct elements, and by means ofa suitably programmed computer. In the device claim enumerating severalmeans, several of these means may be embodied by one and the same itemof hardware. The mere fact that certain measures are recited in mutuallydifferent dependent claims does not indicate that a combination of thesemeasures cannot be used to advantage.

1. An electrophoretic multi-color display device comprising: a pluralityof cells with an electrophoretic medium, pixel electrodes for selectinga subgroup of cells, a color filter array associated with the pixelelectrodes, the color filter array and the pixel electrodes beingprovided at the same side of the electrophoretic medium, wherein aninsulating material with a relatively low refractive index is providedbetween the pixel electrodes and the color filter array and wherein theinsulating material is selected from the group formed by afluor-polymer, a low-dielectric inorganic film and a low-dielectricnano-porous film.
 2. The electrophoretic multi-color display device asclaimed in claim 1, wherein the cells are arranged according to a matrixand in that each cell of the color filter array registers with a pixelelectrode.
 3. The electrophoretic multi-color display device as claimedin claim 1, wherein the cells are arranged according to a matrix and inthat the cells of the color filter array are disposed along linesregistering with the pixel electrodes.
 4. The electrophoreticmulti-color display device as claimed in claim 1, wherein the pixelelectrodes are provided between the cells and the color filter array. 5.An electrophoretic multi-color display device as claimed in claim 1,wherein each of the cells is associated with a thin-film transistor. 6.The electrophoretic multi-color display device as claimed in claim 5,wherein the color filter array and the thin-film transistors areprovided on a translucent front window, elements of the color filterarray being arranged adjacent to the thin-film transistors.
 7. Theelectrophoretic multi-color display device as claimed in claim 1,wherein the cell comprises a low-refractive index material with arefractive index n_(li) in the range from 1.0n_(li)≦1.5.
 8. Theelectrophoretic multi-color display device as claimed in claim 7,wherein the refractive index of the low-refractive index material isn_(li)≦1.4.
 9. The electrophoretic multi-color display device as claimedin claim 7, wherein the low-refractive index material is selected fromthe group formed by a fluor-polymer, a low-dielectric inorganic film anda low-dielectric nano-porous film.
 10. The electrophoretic multi-colordisplay device as claimed in claim 1, wherein the electrophoretic mediumis present in a microcapsule.
 11. The electrophoretic multi-colordisplay device as claimed in claim 10, with one microcapsule per cell orwith one microcapsule per sub-cell.
 12. An electrophoretic multi-colordisplay device comprising: a plurality of cells with an electrophoreticmedium, pixel electrodes for selecting a subgroup of cells, a colorfilter array associated with the pixel electrodes, the color filterarray and the pixel electrodes being provided at the same side of theelectrophoretic medium, wherein the electrical contact between the pixelelectrodes and a respective thin-film transistor is provided via cut-outportions in the color filter array.
 13. A method of forming anelectrophoretic multi-color display device, the method comprising theacts of: forming a plurality of cells with an electrophoretic medium,forming pixel electrodes for selecting a subgroup of cells, forming acolor filter array associated with the pixel electrodes, forming aninsulating material with a relatively low refractive index between thepixel electrodes and the color filter array, wherein the insulatingmaterial is selected from the group formed by a fluor-polymer, alow-dielectric inorganic film and a low-dielectric nano-porous film, andwherein the color filter array and the pixel electrodes are formed atthe same side of the electrophoretic medium.
 14. A method of forming anelectrophoretic multi-color display device, the method comprising theacts of: forming a plurality of cells with an electrophoretic medium,forming pixel electrodes for selecting a subgroup of cells, forming acolor filter array associated with the pixel electrodes, wherein thecolor filter array and the pixel electrodes are formed at the same sideof the electrophoretic medium, and wherein the electrical contactbetween the pixel electrodes and a respective thin-film transistor isformed via cut-out portions in the color filter array.